The present invention relates to the production of high frequency coherent electromagnetic radiation such as soft x-rays and, more particularly, to a device and method for producing this radiation in limited frequency bands by frequency multiplication.
The development of masers and lasers circa 1960 stimulated speculation about applications of similar sources of coherent electromagnetic radiation that could operate at much shorter wavelengths, for example, at soft x-ray wavelengths, between about 1 nm and about 10 nm. Such applications include holographic imaging of biological structures, plasma diagnostics and the generation of intense plasmas. More recent advances in other fields have suggested other applications of coherent soft x-rays. For example, a coherent source of x-rays for CT scanning could be operated at a much lower power level tan the incoherent sources now in use, reducing the exposure of the subjects to ionizing radiation. Another application is in the fabrication of microdevices The design rule of devices such as integrated circuits is now limited by a lower bound of about 0.1 microns by several factors, not least of which is that the shortest wavelength radiation that can be used for photolithography is ultraviolet radiation. A coherent source of soft x-rays would help make even shorter design rules feasible. Similarly, a coherent source of soft x-rays would enable much denser storage of information in media such as compact disks. A sufficiently intense coherent beam of soft x-rays can be used as a weapon, or as an industrial cutting tool.
It is known to produce coherent soft x-rays using soft x-ray lasers. The first soft x-ray laser was developed at Lawrence Livermore National Laboratory in 1984. This device, which was described in general terms by Dennis Matthews and Mordecai Rosen in the December 1988 issue of Scientific American, uses the Nova laser both to create a plasma including high-Z ions and to create a population inversion among those ions by a process known as collisional excitation. The laser medium in such a device is inherently transient. Essentially, the mere process of creating the laser medium, by evaporating a metal foil to produce a plasma. also destroys the laser medium. The use of the Nova laser, originally developed for controlled fusion research, to create the laser medium also meant that the device was a large, expensive research tool unsuitable for practical applications. More compact soft x-ray lasers have been developed in recent years, but like their giant ancestor at LLNL, they all rely on inherently self-destructive mechanisms to create a population inversion in a highly ionized plasma.
Two non-self-destructive strategies for the generation of coherent soft x-rays also have been explored. One is the imposition of spatial periodicity on the trajectories of high energy electrons, in free-electron lasers. This requires the use of a massive high-energy accelerator to create the high energy electrons. The other is the use of frequency multiplication media to create higher harmonics of coherent light, such as ultraviolet light, produced by a conventional laser. This generally produces a broad mixture of wavelengths. For example, N. Sarukura et al. reported in Phys. Rev. A Vol 43 p. 1669 (1991) the creation of 9-th to 23-rd order harmonics of light from a KrF laser using helium as a frequency multiplication medium. A review of the technique may be found in A. L""Huiller et al., xe2x80x9cHigh-order harmonics: a coherent source in the XUV rangexe2x80x9d, Journal of Nonlinear Optical Physics and Materials, July 1995, Vol. 4 No. 3, pp. 647-665, which is incorporated by reference for all purposes as if fully set forth herein. More recently, Preston et al. (Phys. Rev. A Vol. 53 p. R31 (1996)) reported obtaining harmonics up to the 35-th using helium as a frequency multiplication medium.
There is thus a widely recognized need for, and it would be highly advantageous to have, a compact, portable, reusable source of coherent, relatively monochromatic soft x-rays.
According to the present invention there is provided a device for producing high frequency radiation, including: (a) a source of elliptically polarized radiation; and (b) a frequency multiplication medium including at least one constituent having approximate finite symmetry including an axis of approximate Cn symmetry, wherein n is at least 3, and wherein the at least one constituent is oriented so that the elliptically polarized radiation includes an electrical field that is circularly polarized in a plane perpendicular to the axis.
According to the present invention there is provided a method of producing high frequency radiation, including the steps of: (a) providing a frequency multiplication medium including at least one constituent having approximate finite symmetry including an axis of approximate Cn symmetry, wherein n is at least 3; and (b) directing elliptically polarized radiation at an angle to the axis such that the elliptically polarized radiation has an electric field that is circularly polarized in a plane perpendicular to the axis.
According to the present invention there is provided a device for producing high frequency radiation, including: (a) a source of elliptically polarized radiation; and (b) a frequency multiplication medium including a multiplicity of constituents sharing a common approximate shape and a common orientation aligned so that the elliptically polarized radiation has an electrical field that is circularly polarized in a plane perpendicular to the common orientation, each of the constituents having a longest dimension of at least about 6 xc3x85.
According to the present invention there is provided a method of producing high frequency radiation, including the steps of: (a) providing a frequency multiplication medium including a plurality of constituents sharing a common approximate shape and a common orientation, each of the constituents having a longest dimension of at least about 6 xc3x85; and (b) directing elliptically polarized radiation at an angle to the common orientation such that the elliptically polarized radiation has an electric field that is circularly polarized in a plane perpendicular to the common orientation.
An object is said to have an axis of Cn symmetry if the object is invariant under rotations by integral multiples of 2 xcfx80/n about this axis. The full point group of the object can be of higher symmetry than Cn, for example Dn, as long as the object has a Cn axis of symmetry. It is demonstrated in the Appendix that a circularly polarized beam of coherent electromagnetic radiation, incident on a medium whose constituents have commonly oriented Cn symmetry axes, causes the generation and amplification of specific harmonics of the incident beam. If the angular frequency of the incident beam is xcfx89, then the harmonics produced are circularly polarized beams, parallel to the incident beam, of angular frequencies (lnxc2x11)xcfx89, where l is an integer, with the xe2x80x9c+xe2x80x9d beams circularly polarized in the same direction as the incident beam and the xe2x80x9cxe2x88x92xe2x80x9d beams circularly polarized in the opposite direction. So, for example, a circularly polarized beam of a wavelength of 500 nm incident on a medium whose constituents have C5 symmetry axes generates, among others, circularly polarized l=20 harmonics with wavelengths of 5.05 nm (99th harmonic) and 4.95 nm (101st harmonic), in the middle of the soft x-ray band. It should be noted that the scope of the present invention is restricted to the case of nxe2x89xa73, as the case of n=2 reproduces the selection rules of the prior art. In addition, the incident electromagnetic radiation may be elliptically polarized, rather than circularly polarized, as long as the electric field of the incident electromagnetic radiation is circularly polarized in the plane perpendicular to the symmetry axes.
It also is demonstrated in the Appendix that the amplification efficiency of medium grows dramatically with system size, even if the Cn symmetry is not perfect. If the constituents of the medium are large (on an atomic scale) ring-like structures, then the medium produces a range of angular frequencies centered around the frequencies determined by the (lnxe2x89xa71)xcfx89 selection rules, at much higher intensities than are attainable by the prior art methods. In addition, under conditions of only approximate Cn symmetry, the radiation produced is not exactly circularly polarized.
It is important to note that to the extent that the present invention relies on the approximate symmetry of the constituents of the frequency multiplication medium, that symmetry is finite. In other words, the constituents which are brought under the scope of the present invention by virtue of having approximate Cn symmetry axes do not have infinite order (e.g., C∞) symmetry axes. In fact, a frequency multiplication medium made of constituents with parallel C∞ symmetry axes would not function within the scope of the present invention, because the photons of the xe2x80x9chigher harmonicsxe2x80x9d that would be generated would have to have infinite frequency, and therefore infinite energy. Thus, the helium atoms of the prior art frequency multiplication media do not fall within the scope of the present invention because they are spherically symmetrical, and have an infinite number of C∞ axes, along with an infinite number of Cn axes for all finite n. Indeed, the mechanism by which helium atoms function as elements of a frequency multiplication medium is totally different from the mechanism described in the Appendix.
Several types of media may be used for such frequency multiplication of circularly polarized light from sources based on conventional lasers. One such medium consists of a gas of dipolar molecules of Cnv symmetry, which have Cn symmetry axes in the direction of the dipole moments. The molecules are oriented by an externally applied electric field so that all their Cn axes are parallel. Another such medium consists of circular rings of metallic nanoparticles oriented with their approximate Cn axes parallel, for example by having been deposited on a planar substrate. The incident beam is perpendicular to the plane of the substrate. A third such medium consists of parallel carbon nanotubes, which have parallel, exact or approximate Cn symmetry axes, with n typically between 6 and 9. In all three cases, the incident beam is parallel to the exact or approximate Cn axes.
As used herein, the term xe2x80x9cconstituentxe2x80x9d refers to a medium component that, individually, includes the desired exact or approximate C5 axis. For example, the constituents of the C5H5Tl gas that have the desired (exact ) C5 axes are the C5H5Tl molecules themselves; and the constituents of the array of nanoparticle rings that have the desired (approximate) Cn axes are the nanoparticle rings themselves.